Piston ring spacer-expander

ABSTRACT

A spacer-expander for rail-ring type oil control piston ring assemblies with a deformable means secured to the spacer-expander ring and extending radially inwardly thereof for providing a temporary internal diameter for the spacer-expander ring which is as small as or smaller than the root diameter of the piston groove which will receive the ring thereby to prevent overlap of the ends of the spacer-expander ring, to center the piston ring assembly, and to inhibit &#39;&#39;&#39;&#39;pop-out&#39;&#39;&#39;&#39; thereof. The deformable means deforms in a manner which is at least partially resilient and is degradable upon being subjected to engine operating conditions.

United States Patent [72] Inventor Philip L. Bond Richmond, Ind.

[21 Appl. No. 14,458

[22] Filed Feb. 26, 1970 [45] Patented Aug. 24, 1971 [73] Assignee DinaCorporation Toledo, Ohio [54] PISTON RING SPACER-EXPANDER l5 Claim 6Drawing Ftp.

[52] US. Cl 277/140, 277/9.5 [51] Int. Cl F16] 9/06 [50] Field Search277/138, 139,140, 9.5, 9,11

[56] References Cited UNITED STATES PATENTS 3,103,365 9/1963 Callerfelt3,472,521 10/1969 Nisper et a1 Primary Examiner-Robert 1. SmithAttorneys-Walter E. Pavlick, Harold D. Shall and John F.

Teigland ABSTRACT: A spacer-expander for rail-ring type oil controlpiston ring assemblies with a defonnable means secured to thespacer-expander ring and extending radially inwardly thereof forproviding a temporary internal diameter for the spacer-expander ringwhich is as small as or smaller than the root diameter of the pistongroove which will receive the ring thereby to prevent overlap of theends of the spacer-expander ring, to center the piston ring assembly,and to inhibit pop out thereof. The deformable means deforms in a mannerwhich is at least partially resilient and is degradable upon beingsubjected to engine operating conditions.

Pmmmusmn 3.601.415

FIG. 5

l N VENT? PHILIP L. BOND M 9. MM

ATTORNEY PISTON RING SPACER-EXPANDER BACKGROUND OF THE INVENTION Manyvarious types of spacer-expander rings for use in an oil ringassemblyexist in the art. A pair of side rails are utilized with thespacer-expander ring, with the rails positionedon opposed axial sides ofthe spacer-expander and a portion of the latter engaging the radiallyinner side of the rails so that, when the oil ring assembly is disposedin the groove of a piston and the latter is disposed in an enginecylinder, the spacer-expander is circumferentially compressed while itaxially spaces the rails and, since it is resilient and tends tocircumferentially expand, it urges the rails radially outwardly againstthe cylinder wall.

The prior art oil rings of the above type have provided much difficultyin the assembly thereof, since, for assembly purposes, both therails andthe spacer-expander are radially split so that each presents a splitannular configuration with two adjacent circumferential ends. This splitconfiguration of the spacer expander introduces an assembly problem inthat,

It is another object of this invention to provide such a spacer-expanderwherein the deformable means also centers the spacer-expander after thesame and the rails associated therewith have been inserted in the pistonring groove of a the free operation of the coil control ring subsequentto the degrading thereof.

SUMMARY OF THE INVENTION A spacer-expander ring according to thisinvention is adapted to be utilized with a pair of cylinder engagingrails to thereby comprise a rail-ring type oil control ring which ispositioned in the piston ring groove of a piston. With the ends of thespacer-expander in abutment, the inner diameter thereof is greater thanthe root diameter of the piston ring groove.

Deformable means made of a material which is both resilient since it isinstalled in the oil ring groove first and subsequently the rails areinstalled and, further, since the inner diameter of the spacer-expanderwhen its ends are in abutment is greater than the root diameter of thepiston groove, the adjacent circumferential ends of the spacer-expanderhave a tendency to overlap each other prior to and during the assemblyof the rails in the groove. This end overlap results in the rails, wherethey overlie the overlapped ends, projecting excessively from the oilring groove. Once the rails and the spacer-expander are properlyassembled in the oil ring groove, the rails engagement with thespacer-expander will inhibit the overlapping thereof.

An additional problem with prior art oil rings of the above type is thatafter the spacer-expander and side rails have been assembled into theoil ring groove of the piston, and with the ends of the spacer-expanderin proper abutment, the inner diameter of the spacer-expander is stilllarger than the root diameter of the oil ring groove. Accordingly, thespacer-expander and rails can move radially relative to the piston andthereby assume a noncentralized position and in many cases theeccentricity is sufiicient to allow pop-out of the assembly.

Many prior art means have been devised for inhibiting overlapping of thecircumferential ends of the spacer-expanders and include providingradially elongated shoulders on the ends.

adapted to abut each other to inhibit such overlap. However, the radialsize of the shoulders is limited, since they cannot project radiallybeyond the rails, and therefore they do not absolutely inhibit overlapand when overlap does occur it results in any even more dilateriouscondition since the elongated shoulders extend the spacer-expander aneven greater radial amount than would be the case in the absence of theshoulders.

Another prior art method of preventing overlap comprises a mechanicalinterlock for the ends of the spacer-expander. These interlocks inoperation require substantially exact alignment for engagement and onceengaged are somewhat difficult to disengage. Additionally, once theinterlock is engaged, even if only inadvertently during handling, it isoften damaged or destroyed by a subsequent disengagement.

Yet another prior art method of preventing overlap is shown in US. Pat.3,427,014, which issued on Feb. ll, 1969, wherein, while overlap canoccur, nesting of the overlapped portions of the spacer-expander areprevented by utilizing antinesting inserts. However, these. inserts,like the above described radially elongated shoulders, do not preventoverla It is, therefore, an object of this invention to provide aspacer-exapnder for a rail-ring type oil control piston ring assemblywhich includes deformable means for inhibiting the circumferential endsof the spacer-expander from becoming overlapped during the assemblyoperation;

and crushable is secured to the spacer-expander with a portion thereofextending radially inwardly of the spacer-expander thereby to provide anefiective inner diameter of the subassembly of the spacer-expander anddeformable means which is as small as or smaller than the root diameterof the groove. Since this subassembly has a diameter at least as smallas the root diameter of the groove, when this subassembly is placed inthe piston ring groove the free length of the spacer-expander is suchthat the free ends thereof would just be in abutting engagement or beslightly spaced so that the circumferential extent of thespacer-expander is insufficient to allow the ends thereof to becomeoverlapped. Further, since the deformable means'can be resiliently andcrushably deformed, it is deformable so that the spacer-expander can becircumferentially and radially compressed to allow the rails to beassembled thereon and to be deformed still further to allow the pistonwith the spacer-expander and rails assembled thereon to be inserted intothe usual receiving cylinder.

By providing the deformable means in at least three circumferentiallyspaced positions, the same centers the ring-rail assembly with respectto the root of the piston ring groove so that pop-out" is inhibitedprior to the piston being inserted in the cylinder.

FIG. 1 is a plan view of an oil ring assembly having a spacerexpanderring incorporating this invention, various portions of the upper railbeing broken away to better illustrate this spacer-expander and portionsof the spacer-expander being shown in section to better illustrate thedeformable means.

FIG. 2 is a fragmentary enlarged vertical section through a pistonshowing the spacer-expander ring of this invention as installed in theoil ring groove of the piston; this view of the spacer-expander beingtaken on the line 2-2 in FIG. 1.

FIG. 3 is a view like FIG. 2 with the view of the spacer-expander beingtaken on the line 3-3 in FlG. 1.

FIG. 4 is a fragmentary enlarged vertical section through a pistonshowing the spacer-expander of this invention and associated side railsas installed in the oil ring groove of the piston; this view of thespacer-expander and the associated rails being taken on the line 44 inFIG. 1.

FIG. 5 is a fragmentary enlarged vertical section through a piston andan associated cylinder showing the spacer-e pander of this invention andassociated rails as installed in the oil ring groove of the piston andthe piston being inserted in the cylinder; this view of thespacer-expander and the associated rails being taken on the line 4-4 inFIG. 1.

FIG. 6 is a plan view of a portion of a different spacer-expander ringhaving a deformable means secured thereto in accordance with thisinvention.

Referring now to FIGS. 1-5, a piston oil ring assembly 10 comprises apair of annular rails 12 and 14 and a combined or annular oil ringgroove 18 formed in a piston 20 and confined within a cylinder 22 of aninternal combustion engine. The spacer-expander 16 is disposed axiallyintermediate the rails 12 and 14 and is axially spaced from the piston20 thereby.

Each of the rails 12 and 14 is preferably made from a flat strip ofmetal, such as spring steel, and is provided with a rad ially extendinggap 13 to permit it to expand and contract in a radial direction whilethe circumferential length thereof remains substantially constant. Itshould be noted that in the assembled condition of the piston ringassembly 10, when the same is disposed in the groove 18 of the piston 20and confined within the'cylinder 22, the ends 25 and 27 of each of therails 12 and 14, which border the gap 13 therein, still remaincircumferentially spaced from each other, and the rails are radiallyspaced from the root 19 of the groove 18 so as to permit the radialmovement of the rail. Accordingly, each rail is resilient and, byitself, exerts some outward pressure against the wall of the cylinder22. Further, it should be noted that in order to assemble the rails 12and 14 within the groove 18 it is necessary to radially expand the same.Additionally, when the piston 20 is confined within the cylinder 22, thelatter will radially compress the rails 12 and 14.

The spacer-expander 16 comprises a plurality of circumferentially spacedU-shaped members 28, with the opening of the U being radially inwardlyof the ring. The members 28 are arranged in adjacent pairs, with thepaired members connected at their axially upper and also at their lowerradially inner ends by a separate circumferentially extending innersupport member 26. The pairs of adjacent members 28, 28 and their innersupport members 26, 26 being indicated by the numeral 29. Each pair ofmembers 29 is connected by a circumferentially extending outer supportmember 30 to the adjacent pair of members 29; the support memberconnecting the medial portion of the radial outer extremity of a member28 of one pair of members 29 to the adjacent member 28 of the adjacentpair of members 29.

While the spacer-expander 16 is of substantially annular configuration,the same is of greater circumferential length than the rails 12 and 14and is formed with a pair of juxtaposed circumferential ends 36 and 38.The spacer-expander 16 is made from resilient sheet metal, which ispunched and folded and is circumferentially compressible and expansible,and, when the same is operatively confined within the cylinder 22 withthe ends 36 and 38 in engagement, (see FIG. it is circumferentiallycompressed and tends to expand and thereby increase its circumferentialdimension. Such increase in circumferential dimension is accompanied byan increase in diameter and causes the spacer-expander 16 to exert aradially outward pressure against the inner peripheries 33 of the rails12 and 14, thereby forcing the latter against the cylinder wall 22 withan urging force which adds to the urging force supplied by the resilientqualities of the rails 12 and 14 themselves. It should also be notedthat when the oil ring is compressed with the cylinder 22, the innerdiameter of the spacerexpander 16 is greater than the root diameter ofthe groove 18. Accordingly, the spacer-expander is of the nonbottomingtype.

Referring to FIGS. 1, 2 and 3, deformable means in the form of aplurality of circumferentially spaced blocks 40 are secured to thespacer-expander 16. More particularly, the

blocks 40 are made from a resilient material which will rapidly degradewhen exposed to engine operating conditions and will be more fullydescribed hereinafter.

Prior to being associated with the spacer-expander 16, each block 40 hasa free shape such that it can be inserted into the U-shaped members 28from the radially inner end thereof and be slightly resilientlydeformed, by being axially compressed as seen in FIGS. 2 and 3, duringsuch insertion so as to be retained therein. Preferably the free shapeof the block 40 is that of a parallelepiped with its planar surfacesrectangular and, as seen in FIG. 1, the chordal length of the block isgreater than its radial or axial dimension (radial and axial being usedin the sense of the orientation of the spacer-expander in the piston20). It should be understood that shapes other than a parallelepiped canbe utilized for the block 40 as will be apparent in the discussion ofFIG. 6 hereinafter.

The block 40 is inserted into the spacer-expander so that itscircumferentially spaced leading edges 40a and 4012 can contact theinner surface 42 (see FIG. 3) of the base of a first and a secondcircumferentially spaced U-shaped member 28, while the portion of theblock intermediate the edges 40a and 40b is spaced from the innersurface of the U-shaped members 28 adjacent thereto and intermediate thefirst and second U- shaped members. Thus, the medial-radially innersurface 400 of the block 40 extends radially inwardly from the radialinner edge of the inner support members 26, 26 a greater distance thandoes the remainder of the block 40; however, throughout thecircumferential extent of the block, at least a portion thereofextendsradially inwardly from the inner support members 26, 26.

In the preferred arrangement, a block 40 is inserted into thespacer-expander 16 adjacent to each end 36 and 38 thereof, a third blockis inserted diametrically opposed to the ends 36 and 38, and fourth andfifth diametrically opposed blocks are inserted at locations from thethird block. Thus, when the spacer-expander 16 is placed in the groove18 of the piston 20, and prior to the assembly of the rails 12 and 14thereon, as seen in FIG. 2, the medial-radially inner surface 400 of theblock 40 contacts the root 19 of the groove 18 while thecircumferentially opposed ends of the radially inner surface of theblock 40, as seen in FIG. 3, are spaced from the root of the groove 18.At this time, prior to the assembly of the rails 12 and 14, the blocks40 are not radially deformed.

Accordingly, the medial-radially inner surface 40c of the blocks 40provide the inner diameter for the subassembly comprised of thespacer-expander l6 and the blocks 40, which inner diameter, as comparedto the diameter of the root 19 of the piston ring groove 18, is suchthat the ends 36, 38 of the spacer-expander will be just touching orslightly spaced, and the circumferential length of the spacer-expanderis insuffcient for the ends 36 and 38 to become overlapped.Additionally, the blocks 40 radially centralize the position of thespacer-expander in relation to the groove 18. Without the blocks 40,since the inner diameter of the spacer-expander 16, itself, is greaterthan the root diameter of the groove 18, the length of thespacer-expander would be great enough for the ends 36 and 38 to overlap,and the spacer-expander could shift radially relative to the groove.

Referring now to FIG. 4, wherein the rails 12 and 14 are shown assembledonto the spacer-expander 16 and into the groove 18, the inherentresiliency of the rails 12 and 14 exert some radially inwardly pressureagainst the spacer-expander 16 and resiliently deform the medial portion400 of the blocks 40 into the shape seen in FIG. 4. At this time, thecircumferentially spaced ends of the radially inner surface of theblocks 40 move into engagement with the root 19 of the groove 18 and areresiliently deformed, but to a much smaller degree than the medialportion 40c.

The relationship of the parts seen in FIG. 4 is that which exists priorto the piston with the oil ring assembly 10 assembled therein beinginserted into the cylinder. At this time the engagement of the blocks 40with the root 19 of the groove 18 radially centralizes the oil ringassembly 10 relative to the piston 20 so that no pop-out ofthe rails 12and 14 will occur.

Referring now to FIG. 5, the piston 20 and the oil ring assembly 10 areshown after the same have been inserted into the cylinder 22 of aninternal combustion engine. Here the rails 12 and 14 have been radiallycompressed by their engagement with the cylinder 22 to thereby furtherforce the spacerexpander 16 radially inwardly. This radial compressionof the spacer-expander 16 results in a circumferential compressionthereof. Simultaneously, the medial portion 400 of the blocks 40 aredeformed to the shape seen in FIG. 5, while the radially innercircumferentially spaced end portions of the blocks 40 are also furtherdeformed, but not to the extent that the medial portion 400 is deformed.

In FIG. 6, a fragment of a spacer-expander 16a is shown having adeformable means in the form of a block 50 secured to the radially innersurface thereof. Here the radially inner surface 50a of the block 50 isof arcuate shape to conform to the root of a piston ring groovethroughout its length, while its radially outer periphery 50b is formedto conform to the radially inner surface of the spacer-expander 16a. Thespacer-expander 16a is shaped differently than the spacer-expander 16seen in FIG. 1, and comprises a series of portions 52 which are U-shapedwhen seen in the plan view shown in FIG. 6, with the portions 52 beinginterconnected by circumferentially extending support members 54. Theprojections 500 on the inner surface of the block 50, which are receivedinto the U-shape portions 52 are dimensioned so as to be a securedpress-fit thereinto or may be suitably secured by a small amount ofadhesive.

A preferred material from which to make the blocks 40 and 50 has beenfound to be a plastic known as closed celled polystyrene foam. This is athermo plastic material which, while being fairly rigid, is resilient innature and, when its elastic limit is nominally exceeded, plastic andmechanical deformation will take place to relieve the stress andsimultaneously with and subsequent to the plastic and mechanicaldeformation, elastic deformation can take place. This allows a block 40made of polystyrene foam to be inserted into the spacer-expander 16 todeform yet still be securedly attached in the spacer-expander, and alsoallows the oil ring to be assembled into the groove 18 with the blocksresiliently centralizing the same.

A suitable source for the polystyrene foam has been found to be the DowChemical Company of Midland, Mich., United States of America, who marketa variety of suitable types of polystyrene foam under the brand nameSTYROFOAM. The various types of STYROFOAM brand polystyrene material arethermo plastic, havea closed cellular structure, have density in thegeneral range of about 1 to 5 lbs. per cubic foot and, while havingexcellent resistance to water, rapidly degrade in the presence ofgasoline, and also degrade, although at a slower rate in roomtemperature lubricating oil.

One type of the STYROFOAM brand polystyrene plastic which is identifiedby the manufacturer as type HD300 available in extruded form from whichthe blocks 40 or 50 may easily be cut to size, and has providedexcellent results when utiliged in this invention and is easily andinexpensively fabricated and assembled. Type HD300 polystyrene has anaverage den sity of 3.3 lbs. per cubic foot, a compressive strength of120 p.s.i., tensile strength of 225 p.s.i., shear strength of 65 p.s.i.and compressive modulus of 6000 p.s.i.

STYROFOAM brand of polystyrene foam has a heat distortion point ofapproximately 170 F. and above this point, softening begins andmechanical strength decreases. It has been found that at temperatures inthe general range of engine operating temperature at the location of theoil ring assembly 10, i.e., approximately 190 to 200 F., the plasticshrinks and, since the cellular structure thereof collapses, the volumeof shrinkage is substantial. Thus, at engine operating conditions, thetemperature, the presence of gasoline and oil vapors, and an oil filmitself, all of which are present in the vicinity of the oil ring grooveof the piston, cause the blocks 40 or 50 to rapidly degrade and, oncedegraded, the same are carried away in the oil which moves through theoil ring groove in a well known manner.

This oil carries the degraded plastic into the engine sump where iteither settles to the bottom or else is circulated by the coil pump andremoved by the engine filter. Testing has revealed that, after only ashort period of engine operation, the blocks 40 have completelydisappeared from the oil ring grooves and no adverse effect upon engineoperations have resulted from the degraded plastic.

Subsequent to the insertion of the piston 20, with theoil ring assembly10 therein, into the cylinder 22, there is no further use for the blocks40. Once the blocks. 40 have degraded, the oil ring assemblies 10 arefree to move in a normal manner relative to the piston 20. It should beunderstood that due to the deformable nature of the blocks 40, evenprior to becoming degraded, they do not interfere with the normalfunction of the oil ring assemblies 10.

Although preferred embodiments of this invention have been shown anddescribed, changes and modifications can be made therein withoutdeparting from the scope of this invention.

What is claimed is:

1. A circumferentially compressible and expansible piston ring elementof the nonbottoming type for use in the piston ring groove of a piston,said element including a metallic portion and a deformable portion, saidmetallic portion having an annular shape and having a pair ofcircumferentially abuttable terminal ends, said metallic portion, whenassembled in a piston ring groove, having an internal diameter which isgreater than the root diameter of the piston ring groove, saiddeformable means being secured to said metallic portion and projectingradially inwardly therefrom a sufficient extent so that saidpiston ringelement has an internal diameter which is at least as small as the rootdiameter of the piston ring groove into which it will be assembled, saiddeformable portion being rapidly degradable when exposed to engineoperating conditions.

2. A piston ring element according to claim 1 wherein said piston ringelement is a spacer-expander for a combined railring type oil controlpiston ring.

3. A piston ring element according to claim 1 wherein said deformablemeans is made from a resilient plastic material.

4. A piston ring element according to claim 3 wherein said resilientmaterial is a closed cell polystyrene foam.

5. A piston ring element according to claim 4 wherein said polystyrenefoam material has a density of about 3.3 pounds per cubic foot, acompressive strength of about p.s.i., a tensile strength of about 225p.s.i., a sheer strength of about 65 p.s.i. and a compressive modulus ofabout 6000 p.s.i.

6. A spacer-expander for spacing and outwardly urging a pair of rails ina piston oil ring comprising a parted circumferentially compressible andexpansible metallic ring of the nonbottoming type, with the ends of saidparted ring in abutting engagement said parted ring having an internaldiameter which is greater than the root diameter of a piston ring grooveadapted to receive the same, and a plurality of deformable memberssecured to said parted ring at spaced circumferential locations andprojecting radially inwardly therefrom a sufficient extent so that saidspacer-expander has an internal diameter which is at least as small asthe root diameter of a piston ring groove into which it will beassembled, said deformable means being rapidly degradable when exposedto engine operating conditions.

7. A spacer-expander according to claim 6 wherein said deformable meansis made from a plastic material adapted to resiliently and plasticallydeform when compressed and, when exposed to heat in excess of F. andlubricating oil, rapidly degrades.

8. A spacer-expander according to claim 7 wherein said deformable meansis made of foamed polystyrene material.

9. A circumferentially compressible and expansible spacerexpander,having a metallic portion with a split annular configuration which hasan internal diameter, when its ends are placed in abutting relationship,which is greater than the root diameter of a piston ring groove intowhich it is adapted to be installed, and a plurality of deformableplastic elements secured to said metallic portion and extending radiallyinwardly therefrom a sufficient amount such that when saidspacer-expander is disposed in the piston ring groove, it has aneffective internal diameter substantially equal in size to the rootdiameter of the groove.

10. A spacer-expander according to claim 9 wherein said plastic elementsare rapidly degradable when exposed to engine operating conditions.

. 11. A spacer-expander according to claim 10 wherein said plasticelements are made of'foamed polystyrene.

the cylinder of an engine comprising in combination, a piston 1 havingan oil ring groove therein, a combined rail-ring oil control ringdisposed in said oil ring groove, and easily deformable means disposedwithin said groove and radially compressed between the root of saidgroove and said oil control ring for radially centering said oil controlring relative to the root of said groove.

14. An assembly according to claim 13 wherein said deformable means ismade from a resilient material.

15. An assembly according to claim 13 wherein said deformable means israpidly degradable upon being exposed O -to engine operating conditions.

1. A circumferentially compressible and expansible piston ring elementof the nonbottoming type for use in the piston ring groove of a piston,said element including a metallic portion and a deformable portion, saidmetallic portion having an annular shape and having a pair ofcircumferentially abuttable terminal ends, said metallic portion, whenassembled in a piston ring groove, having an internal diameter which isgreater than the root diameter of the piston ring groove, saiddeformable means being secured to said metallic portion and projectingradially inwardly therefrom a sufficient extent so that said piston ringelement has an internal diameter which is at least as small as the rootdiameter of the piston ring groove into which it will be assembled, saiddeformable portion being rapidly degradable when exposed to engineoperating conditions.
 2. A piston ring element according to claim 1wherein said piston ring element is a spacer-expander for a combinedrail-ring type oil control piston ring.
 3. A piston ring elementaccording to claim 1 wherein said deformable means is made from aresilient plastic material.
 4. A piston ring element according to claim3 wherein said resilient material is a closed cell polystyrene foam. 5.A piston ring element according to claim 4 wherein said polystyrene foammaterial has a dEnsity of about 3.3 pounds per cubic foot, a compressivestrength of about 120 p.s.i., a tensile strength of about 225 p.s.i., asheer strength of about 65 p.s.i. and a compressive modulus of about6000 p.s.i.
 6. A spacer-expander for spacing and outwardly urging a pairof rails in a piston oil ring comprising a parted circumferentiallycompressible and expansible metallic ring of the nonbottoming type, withthe ends of said parted ring in abutting engagement said parted ringhaving an internal diameter which is greater than the root diameter of apiston ring groove adapted to receive the same, and a plurality ofdeformable members secured to said parted ring at spaced circumferentiallocations and projecting radially inwardly therefrom a sufficient extentso that said spacer-expander has an internal diameter which is at leastas small as the root diameter of a piston ring groove into which it willbe assembled, said deformable means being rapidly degradable whenexposed to engine operating conditions.
 7. A spacer-expander accordingto claim 6 wherein said deformable means is made from a plastic materialadapted to resiliently and plastically deform when compressed and, whenexposed to heat in excess of 170* F. and lubricating oil, rapidlydegrades.
 8. A spacer-expander according to claim 7 wherein saiddeformable means is made of foamed polystyrene material.
 9. Acircumferentially compressible and expansible spacer-expander, having ametallic portion with a split annular configuration which has aninternal diameter, when its ends are placed in abutting relationship,which is greater than the root diameter of a piston ring groove intowhich it is adapted to be installed, and a plurality of deformableplastic elements secured to said metallic portion and extending radiallyinwardly therefrom a sufficient amount such that when saidspacer-expander is disposed in the piston ring groove, it has aneffective internal diameter substantially equal in size to the rootdiameter of the groove.
 10. A spacer-expander according to claim 9wherein said plastic elements are rapidly degradable when exposed toengine operating conditions.
 11. A spacer-expander according to claim 10wherein said plastic elements are made of foamed polystyrene.
 12. Aspacer-expander according to claim 11 wherein said metallic portion iscomprised of a plurality of U-shaped segments which open radiallyinwardly, said plastic elements are inserted into said segments from theopen end thereof, said elements have an axial thickness in the freestate which is greater than the axial extent of the open end of saidsegments and are axially compressed upon insertion therein, and saidplastic elements extend radially inwardly from said segments.
 13. Apiston and oil ring assembly adapted for insertion in the cylinder of anengine comprising in combination, a piston having an oil ring groovetherein, a combined rail-ring oil control ring disposed in said oil ringgroove, and easily deformable means disposed within said groove andradially compressed between the root of said groove and said oil controlring for radially centering said oil control ring relative to the rootof said groove.
 14. An assembly according to claim 13 wherein saiddeformable means is made from a resilient material.
 15. An assemblyaccording to claim 13 wherein said deformable means is rapidlydegradable upon being exposed to engine operating conditions.